As reduction in size and weight of mobile devices with higher performance proceeds, there are increasing demands for higher capacity and longer cycle life of secondary battery. With this in the background, as a second battery for small portable appliances such as cellular phones and video cameras, lithium secondary batteries such as cylindrical or prismatic lithium ion battery using non-aqueous electrolyte solution and lithium polymer battery, because of their high energy density and high voltage characteristics, have been widely used in many appliances.
As cathode active material used in these lithium secondary batteries, metal oxide compounds such as lithium cobaltate having a high charge-discharge capacity per unit at a high potential are used and as anode active material, carbon materials such as graphite which has a high charge-discharge capacity per unit near that of lithium at a low potential are used.
Conventionally, as anode material, natural graphite, artificial graphite, low-crystalline carbon material, noncrystalline carbon material, surface-coated carbon material, mesophase pitch carbon fiber, carbon material doped with other elements such as boron and the like material have been used so far.
Initially, natural graphite drew attention for its ability to achieve high battery capacity, however, due to the critical problem of its short cycle life caused by strong decomposition reaction of electrolyte solution, practical application of natural graphite was difficult.
On the other hand, artificial graphite which can be obtained by thermal treatment using as raw material coke or the like has relatively good cycle characteristics and therefore, it is being widely used as anode active material.
With a view to obtaining an anode active material with a higher battery capacity and a longer cycle life, studies are being vigorously continued at the present day. For example, those granulated or processed into a spherical shape by subjecting highly-crystalline graphitic material to machinery treatment have been proposed, and studies on anode active materials whose surface is coated with pitch or resin and thermally treated to thereby control surface reactivity are being made.
On the other hand, for maintenance and enhancement in electroconductivity between anode active materials, addition of electroconductive carbon material such as carbon black, graphite fine powder, carbon fiber or vapor grown carbon fiber is effective. Especially, vapor grown carbon fiber, which is a fine fibrous substance, is effective for formation of conductive paths between active materials and in a case where a large current is passed, since vapor grown carbon fiber can reduce electric resistance of electrode, it has been assumed that use of vapor grown carbon fiber is advantageous to produce a large energy. Moreover, in terms of charge-discharge cycle life, it can be assumed that, thanks to its fibrousness, even when active material itself swells or shrinks, the conductive paths can be maintained and therefore, studies have also been made with a view to improvement in cycle life.
Japanese Patent No. 3033175 describes that addition of less than 5% by weight of vapor grown carbon fiber has no effects of enhancing cycle life. However, too large an amount of vapor grown carbon fiber causes significant deterioration of coatability. Further, the more vapor grown carbon fiber is added, the smaller the proportion of the active material accounting for, resulting in decrease in battery capacity and therefore, it is necessary to obtain an effect with a smaller amount of vapor grown carbon fiber.
In Japanese Patent Application Laid-Open No. 2000-133267, cycle life is improved by adding from 0.5 to 22.5 parts by mass of vapor grown carbon fiber. The document describes a technical feature that in the electrode, secondary particles comprising vapor grown carbon fiber having an average particle size of 12 to 48 μm are contained. However, in an electrode having such a condition, no enhancement in cycle life characteristics was observed (Comparative Example 7). The reason can be assumed to be that when vapor grown carbon fiber is localized, electric current converges onto the secondary particles, resulting in concentrated deterioration of the portion. Thus, further improvement is required.
Recently, with an aim to improve large-current characteristics and cycle life, a material which comprises carbon fiber grown directly from a surface of an anode active material has been reported. (Japanese Patent Application Laid-Open No. 2004-250275). Its effects include improvement in large-current characteristics. However, given that the discharge capacity over 5-hour discharge time is 100%, the ratio of discharge capacity over discharge time of 20 minutes (with a large current condition where current density is 15 times more) is 88% (Example of Japanese Patent Application Laid-Open No. 2004-250275), there is room for further improvement. The cause can be assumed to be that the degree of crystallization of carbon fiber generated only through chemical deposition is generally low and is too insufficient to impart electroconductivity.